Easiest way is to put pieces of cardboard on the sides to catch the spraying oil so it runs back into the heads, then make the adjustments with the engine running. Back off all the nuts enough so there is no misfire. Clattering is okay for now. Run each nut down enough until that valve stops clattering.

Run one nut down about 1/4 turn at a time until you feel a misfire. It will clear up in a few seconds as that lifter bleeds down. Count how many turns you tighten this nut. Turn the nut another 1/4 turn, then wait again for the misfire to clear up. After going about 2 to 3 turns the misfire will not clear up because the valve is being held open. Back the nut off until the misfire goes away, then back it off one half the number of turns from where the clattering stopped. That will put the lifter in the middle of its range.

The other way takes a lot more time and you'll need a chart that shows which valves to adjust when. You start by turning the engine until piston number 1 is at top dead center on the compression stroke, then the chart will tell which exhaust and which intake valve to adjust. Spin the push rod with your fingers until you feel a slight drag, then, as I recall, turn the nut one more turn. You may still be able to press the rocker arm and push rod down a little because there's a spring inside each lifter. There just won't be any oil in them without the engine running.

Next, you turn the engine 1/4 turn, then adjust the next two valves. You might be able to figure out which valves to do by feel if you don't have the chart. The chart will be in any GM service manual from the '60s and '70s.